1. Field of the Invention
The present invention relates to an optical attenuator, and particularly to a variable optical attenuator having an electrical controlling circuit.
2. Description of the Related Art
Optical attenuators are used to optimize the optical power of signals at key points in optical communications networks. For example, in networks having Erbium Doped Fiber Amplifiers (EDFAs), optical attenuators are used between stages of EDFAs to provide constant gain. In Wavelength Division Multiplexer (WDM) systems, optical attenuators are used to adjust optical power of “added” laser signals to match the signals strength of other channels within the network. Optical attenuators can also be used to set signal strength within the range of a particular receiver.
Known methods to obtain a variable optical attenuator include coating a filter element with an attenuation layer having a variable density, and bending optical fibers to get a given attenuation. A variable optical attenuator can also be obtained by changing a distance between a reflector and an input port or an output port.
U.S. Pat. No. 5,745,634 discloses a voltage controlled attenuator comprising a first lens for receiving an incoming light beam, a second lens for outputting the attenuated light beam, an optical detecting means and a controllable attenuating means. The optical detecting means monitors the intensity of the attenuated light beam, and the controllable attenuating means varies the attenuation of the outgoing light beam in response to signals from the detecting means. However, making the attenuating means (such as variable neutral density filters or wedge shaped filters) is very difficult. Furthermore, when a light beam passes through the filters, the reflections of the light beam at the interfaces of the filters can cause the intensity of the outgoing light beam to fluctuate.